Method and compositions for fracturing subterranean formations

ABSTRACT

An aqueous gel containing a retarded crosslinking composition comprising a zirconium salt or chelate and a polyhydroxyl-containing compound and optionally including an aqueous fluid or an alkanol. The gel is useful for fracturing and placing propping agents within a subterranean formation. The gel has a high viscosity in the formation and has pumping characteristics in turbulent flow similar to those of the base gel.

Background of the Invention

1. Field of the Invention

This invention relates to methods and compositions for the hydraulicfracturing of subterranean formations. It more particularly relates tomethods and compositions for fracturing a subterranean formationpenetrated by a well bore wherein a fluid composition having retardedcrosslinking properties is injected into a formation through a suitableconduit at a rate and pressure sufficient to produce a fracture in theformation.

2. Brief Description of the Prior Art

In the drilling, completion and treatment of subterranean formationspentrated by well bores, viscous treating fluids commonly are utilized.In such operations, it often is desirable or necessary that the viscoustreating fluids have relatively low initial viscosities, but when placedin the well bore or subterranean formation to be treated, theviscosities of the fluids increase. For example, in performing asubterranean fracturing process on a hydrocarbon-bearing formation tostimulate the production of hydrocarbons therefrom, a treating fluidwhich has a low viscosity and a low friction pressure when being pumpedbut which exhibits a high viscosity in the formation is desirable.

Generally, in the art of hydraulic fracturing, a fluid is introducedthrough a conduit, such as tubing or casing, disposed in the well boreinto a formation sought to be fractured. The fluid is introduced at arate and pressure sufficient to produce a fracture or fractures in theformation and to extend the produced fracture or fractures from the wellbore into the formation. Upon the creation of the fracture or fractures,additional fracturing fluid containing solid proppant materials can beintroduced into the fracture or fractures in the event the initial fluiddid not contain any proppant. Following this treatment, the introducedfluid is recovered from the formation, but the proppant remains in theproduced fracture or fractures to thereby prevent the complete closurethereof. The propped fracture creates a conductive channel extendingfrom the well bore into the formation.

The conductivity of a propped fracture is effected by the particle sizeof the proppant material placed in the fracture. The particle size ofthe proppant that can be used depends upon the width to which theparticular fracture can be opened during the introduction of thefracturing fluid. The fracture width normally is directly proportionalto the viscosity of the fracturing fluid. In addition, the use offracturing fluids having relatively high viscosities is advantageoussince such fluids can support the proppant particles suspended thereinwithout excessive settling. The use of such high viscosity fluids alsopermits the placement of relatively large-size proppant material in thefracture without a screenout occurring, that is, without the proppantbridging across the mouth of the fracture and preventing theintroduction of proppant therein.

The use of desirably high viscosity fracturing fluids undesirably isaccompanied by the problem of high friction losses usually encounteredduring the introduction of such fluids into a formation through theconduit, such as tubing or casing, disposed in the well bore. Since thepumping equipment and tubular goods are limited in capacity andoperating pressure, the viscosity of the fluid which can be pumped alsois limited. The viscosity of the fluid must be low enough that excessivefriction losses and high well head pumping pressures are notencountered.

Summary of the Invention

By the present invention there are provided methods of forming and usingan improved viscous treating fluid. The treating fluid has an initialviscosity such that solid proppants can be suspended therein and carriedthereby without excessive settling, but the viscosity of the fluid isnot so high that excessive friction pressures are encountered in pumpingthe fluid. Thus, according to this invention, an aqueous gel is providedwhich contains a retarded crosslinking composition capable of effectinga delayed crosslinking of the gelling agent in the aqueous gelled fluidto produce a fluid of significantly higher viscosity.

Brief Description of the Preferred Embodiment

In accordance with the present invention an aqueous gel is providedcomprising an aqueous fluid, a gelling agent, and a retardedcrosslinking composition which is soluble in the aqueous fluid andcapable of effecting a delayed crosslinking of the gelling agent. Theaqueous gel has a non-Newtonian viscosity in laminar flow. However,during introduction of the aqueous gel into the formation through aconduit in which the fluid is in turbulent flow, the viscosity is nogreater than that imparted by the gelling agent before crosslinking. Theaqueous gel of the present invention can carry great quantities ofproppants into a formation sought to be fractured and can be introducedinto the formation at suitably high rates with pumping equipment andtubular goods normally available at the wellhead.

The aqueous fluid utilized herein is defined as a water-alcohol solutionhaving from 0 to 80 percent and preferably from about 0 to 40 percentand most preferably from about 0 to 10 percent alcohol by volume of thesolution. The preferred alcohols are alkanols having from 1 to 5 carbonatoms. Examples of alcohols believed to be useful in the aqueous fluidinclude methanol, ethanol, propanol, isopropanol, butanol, isobutanol,pentanol, furfuryl alcohol, ethylene glycol, and ethoxylated derivativesthereof.

The aqueous fluid is used to solvate the gelling agent. The solvatedgelling agent is referred to hereinafter as a "base gel". The pH of theaqueous fluid can be adjusted, if necessary, to render the fluidcompatible with the crosslinking agent used to crosslink the solvatedgelling agent. The pH adjusting material can be added to the aqueousfluid before, after, or during addition of the gelling agent to theaqueous fluid.

The gelling agent useful in the present invention is selected fromsolvatable polysaccharides having molecular weights of at least 100,000.Examples of polysaccharides useful herein include the galactomannangums, glucomannan gums, and their derivatives. Solvatable galactomannanand glucomannan gums are naturally occurring. The galactomannan gums andglucomannan gums also can be reacted with hydrophilic constituents tothereby produce gelling agents useful herein.

Solvatable polysaccharides having molecular weights of less than about100,000 do not form crosslinked gels which are useful herein. The mostpreferred solvatable polysaccharides useful herein have molecularweights in the range of from about 200,000 to about 3,000,000.

Guar gum, locust bean gum, karaya gum, sodium carboxymethylguar,hydroxyethylguar, sodium carboxymethylhydroxyethylguar,hydroxypropylguar and sodium carboxymethylhydroxypropylguar are examplesof gelling agents useful herein.

The preferred gelling agents are guar gum, hydroxypropylguar and sodiumcarboxymethylhydroxypropylguar. The most preferred gelling agent ishydroxypropylguar.

The gelling agent useful herein is present in the aqueous fluid in aconcentration in the range of from about 0.2 to about 1.25 percent,preferably from about 0.2 to about 1.0 percent and most preferably fromabout 0.4 to about 0.7 percent by weight of the aqueous fluid. A gellingagent concentration of less than about 0.2 percent by weight of theaqueous fluid is not a sufficient quantity of gelling agent to permiteffective crosslinking of the gel within the formation.

The discovery now has been made that the introduction of a retardedcrosslinking composition comprising an admixture of a crosslinkingcompound and a polyhydroxyl-containing compound to the base gel willprovide a controllable delay in the rate of the crosslinking reaction.This retarded aqueous gel readily can be introduced through a conduitinto a subterranean formation sought to be fractured as a result of itsrelatively low initial viscosity. The significant increase in theviscosity of the gel through crosslinking as it reaches the lowerportion of the conduit or upon entry into the formation facilitates thefracturing process through a reduction in the hydraulic horsepowernecessary to effect the fracture.

The retarded crosslinking composition also can contain an aqueous fluidor an alkanol having from about 1 to 6 carbon atoms. The presence ofcontrolled amounts of the aqueous or alkanol in the retardedcrosslinking composition has been found to further delay the rate of thecrosslinking reaction.

The retarded crosslinking composition of the present invention utilizescrosslinking compounds which feature the presence of zirconium in the +4oxidation state and are referred to as zirconium salts or chelates. Anexample of a zirconium (IV)-containing crosslinking compound usefulherein is zirconium (IV) acetylacetonate chelate which is available fromKay Fries, Rockleigh, N.J. Another example of a zirconium salt orchelate useful herein is zirconium carbonate available from MagnesiumElectron, Starret, Tex. Yet another example is zirconium lactate andzirconium diisopropylamine lactate which are available from ZirconiumTechnology, Gainsville, Fla.

The crosslinking mechanism is not fully understood. However, it isbelieved that the zirconium does not experience any sort of valencechange during the crosslinking reaction.

The amount of crosslinking compound useful to crosslink the gellingagent of this invention is that which provides a zirconium ionconcentration in the range of from about 0.0005 percent to in excess ofabout 0.01 percent by weight of the aqueous gelled fluid. The preferredconcentration is in the range of from about 0.001 percent to about 0.01percent by weight of the aqueous gelled fluid.

The rate of the unretarded crosslinking reaction is extremely rapid. Atambient temperature conditions, the zirconium chelates comprising thecrosslinking compound can crosslink the polysaccharides, comprising thegelling agent in as little as 10 to 15 seconds. When the aqueous fluidof the base gel is maintained at an elevated temperature, such as whenpreheated solutions are employed having a temperature above 100° F., theunretarded crosslinking reaction occurs almost instantaneously uponintroduction of the crosslinking compound into the base gel. Such rapidreaction rates do not permit the gelled fluid to be pumped into thesubterranean formation before a signifcant increase in the viscosity ofthe fluid occurs.

The surprising discovery now has been made that admixing thecrosslinking compound with at least a polyhydroxyl-containing compoundin predetermined amounts produces a composition which can be used todelay the rate of the crosslinking reaction for a period of timesufficient to permit pumping of the aqueous gel through the conduit tothe subterranean formation. The crosslinking compound andpolyhydroxyl-containing compound also can be admixed with an aqueousfluid or a selected alkanol. Typically, this time can be from severalminutes to hours in extremely deep formations.

The polyhydroxyl-containing compound useful in the present invention isselected from the polyhydroxyl-containing compounds having from 3 to 7carbon atoms. Examples of compounds useful herein include glycerol,erythritol, threitol, ribitol, arabinitol, xylitol, allitol, altritol,sorbitol, mannitol, dulcitol, iditol, perseitol, and the like. Thepreferred polyhydroxyl-containing compound for use in the invention isglycerol. The compound can be in solid or liquid form when admixed withthe aqueous and complexing compound of the present invention.

The polyhydroxyl-containing compound useful herein is admixed with anaqueous fluid and the crosslinking compound in an amount sufficient toprovide a controlled delay in the crosslinking rate of the base gel. Theparticular amount of polyhydroxyl-containing compound necessary to delaythe crosslinking reaction will depend upon the specific gelling agent,crosslinking compound and polyhydroxyl-containing compound utilized aswell as the equipment available at the wellhead and tubular goods whichwill affect the pumping rate of the aqueous gel into the formation.

The aqueous fluid utilized to formulate the retarded complexingcomposition can comprise substantially any aqueous solution which doesnot adversely react with the gelling agent, crosslinking compound orpolyhydroxyl-containing compound. Preferably, the aqueous fluidcomprises water.

The alkanol utilized to formulate the retarded complexing compositioncan comprise alcohols having from 1 to 6 carbon atoms. Examples ofalcohols believed to be useful in the composition include methanol,ethanol, propanol, isopropanol, butanol, pentanol, hexanol, ethyleneglycol and ethoxylated derivatives thereof.

The retarded crosslinking composition is prepared by admixing thecrosslinking compound and polyhydroxyl-containing compound inpredetermined amounts. The constituents are admixed in a volumetricratio of crosslinking compound to polyhydroxyl-containing compound inthe range of from about 0.01:1 to about 100:1. Preferably, thevolumetric ratio is in the range of from about 0.1:1 to about 10:1 and,most preferably, the volumetric ratio is about 0.5:1 to about 5:1.

When an aqueous fluid or alkanol is present, the constituents areadmixed in a volumetric ratio of crosslinking compound topolyhydroxyl-containing compound to aqueous fluid or alkanol in therange of from about 1:10:10 to about 1:0.1:0.1. Preferably, thevolumetric ratio is in the range of from about 1:0.5:0.5 to about 1:2:2.

The constituents of the retarded crosslinking composition can be admixedin any order in any conventional mixing apparatus, such as for example,a batch mixer. When an aqueous-containing solution of the crosslinkingcompound is utilized, the aqueous portion is included in determining thetotal aqueous fluid content of the retarded crosslinking composition.The retarded crosslinking composition can be admixed with the aqueousgel in an amount of from about 0.01 gallon to about 0.5 gallon per 10pounds of gelling agent.

Surprisingly, it has been found that the high temperature rheologicalproperties of the aqueous gels formed with the retarded crosslinkingcomposition of the present invention improve when the retardedcrosslinking composition is "aged" prior to use. The term "aged" as usedherein is intended to mean that the admixture comprising the retardedcrosslinking composition is held in an appropriate container afterformulation for a period of from a few minutes to over several weeksprior to use. Preferably, the retarded crosslinking composition is agedfor from about 8 hours to about 25 weeks. It has been found that whenthe retarded crosslinking composition is aged at a generally constanttemperature, the low-temperature crosslinking reaction rate declineswhile the high temperature viscosity of an aqueous gelled fluidcrosslinked with the retarded crosslinking composition increases. Whenthe retarded crosslinking composition is aged at a temperature aboveambient temperature, such as for example, an elevated temperature suchas from about 100° F. to 180° F., the rate of decline in thecrosslinking reaction rate and rate of increase in the high temperatureviscosity of the aqueous gelled fluid are enhanced. This permits theproduction of retarded crosslinking compositions having preselectedproperties by controlling the time and temperature of the aging.

Conventional propping agents can be employed with the fracturing fluidcompositions of the present invention, examples of which are quartz sandgrains, tempered glass beads, rounded walnut shell fragments, aluminumpellets, sintered bauxite, nylon pellets, and similar materials.Propping agents generally are used in concentrations in the range offrom about 1 to about 10 pounds per gallon of the aqueous fluid;however, higher or lower concentrations may be used as required. Theparticle size of propping agent employed is a function of the nature ofthe formation to be fractured, the pressure required to produce thefracture, and pumping fluid flow rates available, as well as other knownfactors. However, particle sizes in the range of from about 200 to about2 mesh on the U.S. Sieve Series scale can be employed in fracturing wellformations with the compositions of the present invention.

The aqueous gel of the present invention can be prepared for use bymixing a predetermined quantity of the solvatable polysaccharide gellingagent with a quantity of aqueous fluid to form a solvated gel. Anyconventional batch mixing apparatus can be employed for this purpose.After the gelling agent and aqueous fluid have been mixed for a timesufficient to dissolve the gelling agent and form the base gel, aquantity of the retarded crosslinking composition is mixed with the gel.The mixture then is pumped into the wellbore and into the formation asthe retarded crosslinking reaction takes place. Proppant generally isadded to the base gel prior to addition of the retarded crosslinkingcomposition as the gel is introduced into the wellbore.

The aqueous gel of this invention can be made over a wide pH range andbe useful for fracturing subterranean formations. The rate at which thecrosslinking reaction proceeds at normal temperatures (about 60° F. toabout 120° F.) is a function of the pH of the base gel. To assure thatthe crosslinking reaction takes place in the desired period of time, thepH of the aqueous fluid or of the base gel can be adjusted to a desiredlevel within the range of from about pH 5.0 to about 11.0 and,preferably, to a level within the range of from about 9 to about 10.5 bythe addition of a pH adjusting chemical. Since water from most sourcesis substantially neutral, the chemical or chemicals used for thispurpose can be acids, acid buffers, mixtures thereof, or mixtures ofacids and bases. Examples of suitable acids are hydrochloric acid,formic acid, acetic acid, fumaric acid, and phthalic acid. Examples ofsuitable buffers are potassium biphthalate, sodium hydrogen fumarate,sodium bicarbonate and sodium carbonate. Examples of mixtures of acidsand bases are fumaric acid and sodium fumarate, adipic acid and sodiumbicarbonate, and fumaric acid and sodium carbonate.

A presently preferred process for fracturing a subterranean formationpenetrated by a well bore comprises injecting down the well bore andinto the formation, at a pressure sufficient to fracture the formation,a fluid comprising an aqueous gel which is prepared by adding from about30 to about 70 pounds of gelling agent comprising hydroxypropylguar toeach 1,000 gallons of aqueous fluid containing about 0 to about 40percent by volume methanol. If desired, the pH of the aqueous fluid canbe adjusted by the addition of a sufficient quantity of a bufferingagent such as fumaric acid, formic acid, sodium carbonate or sodiumbicarbonate. The base gel is introduced into the well bore and, as it isintroduced, a sand proppant is introduced in an amount of from about 1pound to about 10 pounds per gallon and the retarded crosslinkingcomposition then is introduced. The retarded crosslinking composition iscomprised of zirconium (IV) lactate, glycerol and methanol in avolumetric ratio of about 1:1:1 and is introduced at the rate of 0.1 toabout 0.25 gallon per each 10 pounds of gelling agent per each 1,000gallons of aqueous fluid.

After the aqueous gel has been pumped into the subterranean formationand a fracture has been formed, it is desirable to convert the gel intoa low viscosity fluid so that it can be recovered from the formationthrough the well bore. This conversion often is referred to as"breaking" the gel. There are various methods available for breaking theaqueous gel of the present invention. The gels of the present inventionbreak after either or both the passage of time and prolonged exposure tohigh temperatures. However, it is desirable to be able to predictbreaking time within relatively narrow limits. Therefore, breakersoptionally can be included in the crosslinked gel of the presentinvention. Mild oxidizing agents are useful as breakers when a gel isused in a relatively high temperature formation, although formationtemperatures of 200° F. or greater will generally break the gelrelatively quickly without the aid of an oxidizing agent. A suitableoxidizing agent is ammonium persulfate. For crosslinked gels used attemperatures below about 140° F., enzymes are generally used asbreakers. Suitable enzymes for such use are alpha and beta amylases,amyloglucosidase, oligoglucosidase, invertase, maltase, cellulase, andhemicellulase.

To further illustrate the present invention, the following examples areprovided. It is to be understood that the examples are not intended tolimit the scope of this invention.

EXAMPLE I

A base gel is prepared by admixing 50 pounds of hydroxypropylguar per1,000 gallons of 2 percent potassium chloride solution together with 15pounds of sodium carbonate per 1,000 gallons of solution. The pH of thebase gel is adjusted to a pH level of about 10. Several tests then areperformed in which aliquots of the base gel are admixed with quantitiesof the retarded crosslinking compositions identified below in a Waringblender jar. The retarded crosslinking composition was prepared invarious ratios and aged for various periods of time at 80° F. Thecrosslinking compound comprised a solution consisting of zirconium (IV)acetylacetonate, a polyhydroxyl-containing compound and an alkanol orwater. The polyhydroxyl-containing compound comprised glycerol orsorbitol and the alkanol comprised methanol, isopropanol or n-butanol.The retarded crosslinking composition is admixed with the base gel inthe Waring blender jar in a ratio of 0.2-0.4 gallon of retardedcrosslinking composition per 1,000 gallons of solution in the aqueousgel. The base gel is stirred at approximately 2700 rpm in the Waringblender jar to admix the retarded crosslinking composition with the basegel. The time required for the vortex to close and the fluid surface tobecome static was recorded as the crosslink time. The data is set forthin Table I, below.

                                      TABLE I                                     __________________________________________________________________________    Effect of Retarded Crosslinking Composition or Crosslinking time of Base      Gel                                                                                                  Volumetric                                                                    Concentration                                                                         Molar Ratio of                                                        Crosslinking                                                                          Crosslinking                                                   Duration                                                                             Compound.sup.1 :                                                                      Compound:                                      Test                                                                             Polyhydroxyl of Aging,                                                                            Polyhydroxyl:                                                                         Polyhydroxyl:                                                                         Crosslinking Time,                     No.                                                                              Compound                                                                             Alkanol                                                                             (Minutes)                                                                            Alkanol Alkanol (Minutes:Seconds)                      __________________________________________________________________________    1  glycerol                                                                             --    --     95:5:0  1:0.45:0                                                                              instantaneous                          2  "      --    --     90:10:0 1:0.95:0                                                                              instantaneous                          3  "      --    --     85:15:0 1:1.5:0 1:05                                   4  "      --    5      80:20:0 1:2.1:0 4:31                                   5  "      --    15     80:20:0 1:2.1:0 3:17                                   6  "      --    30     80:20:0 1:2.1:0 3:38                                   7  "      --    --     70:30:0 1:3.67:0                                                                              2:50                                   8  "      --    --     50:50:0 1:8.56:0                                                                              1:43                                   9  "      --    --     30:70:0 1:19.96:0                                                                             0:31                                   10 "      --    10     10:90:0 1:77:0  0:22                                   11 "      --    30     10:90:0 1:77:0  0:50                                   12 --     isopropanol                                                                         --     50:0:50 1:0:8   0:27                                   13 glycerol                                                                             "     5      80:10:10                                                                              1:1:1   1:42                                   14 "      "     30     80:10:10                                                                              1:1:1   3:30                                   15 "      "     60     80:10:10                                                                              1:1:1   3:35                                   16 "      "     120    80:10:10                                                                              1:1:1   4:25                                   17 "      "     2,880  80:10:10                                                                              1:1:1   10:45                                  18 "      "     5      40:30:30                                                                              1:6.5:6.5                                                                             1:01                                   19 "      "     60     40:30:30                                                                              1:6.5:6.5                                                                             2:41                                   20 "      "     1,440  40:30:30                                                                              1:6.5:6.5                                                                             8:04                                   21 "      m-butanol                                                                           --     95:2.5:2.5                                                                            1:0.23:0.18                                                                           instantaneous                          22 "      "     --     87.5:10:2.5                                                                           1:1:0.19                                                                              instantaneous                          23 --     "     --     80:0:20 1:0:0.59                                                                              0:13                                   24 glycerol                                                                             "     --     80:10:10                                                                              1:1.08:0.85                                                                           1:25                                   25 "      "     --     80:17.5:2.5                                                                           1:1.9:0.21                                                                            0:44                                   26 "      "     --     50:10:40                                                                              1:1.7:5.4                                                                             4:25                                   27 "      methanol                                                                            --     95:2.5:2.5                                                                            1:0.22:0.41                                                                           instantaneous                          28 "      "     5      90:5:5  1:0.47:0.86                                                                           0:45                                   29 "      "     30     80:10:10                                                                              1:1.08:1.89                                                                           2:39                                   30 "      "     1,440  80:10:10                                                                              1:1.08:1.89                                                                           3:47                                   31 "      "     264                                                                              hours                                                                             80:10:10                                                                              1:1.08:1.89                                                                           11:47                                  32 "      "     30     70:10:20                                                                              1:1.22:4.38                                                                           1:42                                   33 "      "     1,440  70:10:20                                                                              1:1.22:4:38                                                                           4:10                                   34 "      "     144                                                                              hours                                                                             70:10:20                                                                              1:1.22:4.38                                                                           5:30                                   35 "      "     15     75:5:20 1:0.57:4.08                                                                           4:11                                   36 "      "     30     60:10:30                                                                              1:1.43:7.71                                                                           1:42                                   37 "      "     1,440  60:10:30                                                                              1:1.43:7.71                                                                           3:13                                   38 "      "     168                                                                              hours                                                                             60:10:30                                                                              1:1.43:7.71                                                                           5:04                                   39 "      "     30     60:20:20                                                                              1:2.85:5.14                                                                           4:10                                   40 "      "     --     50:10:40                                                                              1:1.71:12.4                                                                           1:17                                   41 "      "     1,440  50:10:40                                                                              1:1.71:12.4                                                                           2:33                                   42 "      "     168                                                                              hours                                                                             50:10:40                                                                              1:1.71:12.4                                                                           9:40                                   43 sorbitol                                                                             "     --     80:10:10                                                                              1:0.64:1.93                                                                           instantaneous                          44 "      "     --     70:10:20                                                                              1:0.73:4.41                                                                           4:58                                   45 "      "     --     60:10:30                                                                              1:0.85:7.72                                                                           8:57                                   46 glycerol                                                                             water 30     80:10:10                                                                              1:1:1   1:15                                   47 "      "     1,440  80:10:10                                                                              1:1:1   7:15                                   38 "      "     2,880  80:10:10                                                                              1:1:1   9:30                                   __________________________________________________________________________     .sup.1 The crosslinking compound was a zirconium (IV) bisacetylacetonate      solution containing 14.38% by weight Zr.sup.+4 in nbutanol.              

EXAMPLE II

A base gel is prepared by admixing 50 pounds of hydroxypropylguar per1,000 gallons of aqueous fluid together with 15 pounds of sodiumcarbonate, 10 pounds of sodium bicarbonate and 20 pounds anhydroussodium thiosulfate per 1,000 gallons of solution. The aqueous fluidcomprises a 2 percent potassium chloride solution containing 5 percentby volume methanol. Aliquots of the base gel are admixed with varyingquantities of the retarded crosslinking composition, and the mixture iscirculated through a Jabsco pump for 4 minutes. The crosslinkingcompound comprises a solution consisting of zirconium (IV)acetylacetonate, glycerol and methanol. The volumetric ratio ofcrosslinking compound comprising 14.38% by weight Zr⁺⁴ in n-butanol topolyhydroxyl-containing compound to alcohol is approximately 8:1:1. Theviscosity of the fluid is measured at the temperature designated in aModel 35 Fann viscometer with a No. 1 spring and standard bob. Theresults of these tests are set forth in Table II, below.

                  TABLE II                                                        ______________________________________                                        Apparent Viscosity Values of Crosslinked Base Gel                                        Retarded                                                           Retarded   Crosslinking            Apparent                                   Crosslinking                                                                             Composition                                                                              Tem-   Total Viscosity,                                 Composition                                                                              Con-       per-   Time  cps                                        Volumetric Ratio                                                                         centration ature  (Min- 170   511                                  and Composition                                                                          (Gal/1000) (°F.)                                                                         utes) sec.sup.-1                                                                          sec.sup.-1                           ______________________________________                                        zirconium (IV)-                                                               bisacetyl-                                                                    acetonate:glyc-                                                               erol:methanol                                                                 7:1:2      0.21        70     0    117    58                                                         80     8    126    63                                                        100    14    167    85                                                        120    19    481   238                                                        140    23    .sup. --.sup.1                                                                      .sup. --.sup.1                                             160    28    --    --                                                         180    36    --    --                                                         180    46    652   385                                                        180    56    633   371                                                        180    66    600   342                                  8:1:1      0.20        70     0    120    60                                                         80     8    119    59                                                        100    12    116    59                                                        120    18    141    70                                                        140    22    174     83                                                       160    28    344   155                                                        180    34    285   140                                                        180    44    215   105                                                        180    54    186    91                                                        180    64    162    80                                  8:1:1      0.375       70     0    120    60                                                         80     9    153    75                                                        100    13    246   115                                                        120    18    304   151                                                        140    24    .sup. --.sup.1                                                                      .sup. --.sup.1                                             160    30    --    --                                                         180    48    --    --                                                         180    58    --    --                                                         180    68    --    --                                                         180    78    675   391                                  ______________________________________                                         NOTE:                                                                         .sup.1 Fluid viscosity exceeded measurement capacity of viscometer       

These tests clearly illustrate the controlled delay which can beeffected by use of the retarded crosslinking composition of the presentinvention.

EXAMPLE III

A base gel is prepared by admixing 70 pounds of hydroxypropylguar per1,000 gallons of aqueous fluid together with 15 pounds of sodiumcarbonate, 10 pounds of sodium bicarbonate and 20 pounds anhydroussodium thiosulfate per 1,000 gallons of solution. The aqueous fluidcomprises a 2 percent potassium chloride solution containing 5 percentby volume methanol. Aliquots of the base gel are admixed with varyingquantities of the retarded crosslinking composition, and the mixture iscirculated through a Jabsco pump for ten minutes. The crosslinkingcompound comprises a solution consisting of zirconium IVacetylacetonate, glycerol and methanol. The volumetric ratio ofcrosslinking compound comprising 14.38% by weight Zr⁺⁴ in n-butanol topolyhydroxyl-containing compound to alcohol is approximately 8:1:1. Theviscosity of the fluid is measured at a temperature of about 350° F. ina Model 50 Fann viscometer with a No. 1 spring and standard bob. Theresults of these tests are set forth in Table III, below.

                  TABLE III                                                       ______________________________________                                        Apparent Viscosity Values of Crosslinked Base Gel                                        Retarded                                                           Retarded   Crosslinking            Apparent                                   Crosslinking                                                                             Composition                                                                              Tem-   Total Viscosity,                                 Composition                                                                              Con-       per-   Time  cp                                         Volumetric Ratio                                                                         centration ature  (Min- 170   511                                  and Composition                                                                          (Gal/1000) (°F.)                                                                         utes) sec.sup.-1                                                                          sec.sup.-1                           ______________________________________                                        8:1:1      .2         117     3    282.5 125.5                                                      350    25    259.9 135                                                        350    40    134.2 72.2                                                       350    55    82.7  44.9                                                       350    85    47.7  25.4                                                       350    100   39.3  20.1                                 8:1:1      .3          92     3    301   135                                                        350    35    536.7 304.7                                                      350    50    351.1 191.0                                                      350    80    163.2 92.6                                                       350    110   81.3  48.6                                                       350    155   35.1  21.7                                 8:1:1      .4          90     3    276.8 123.7                                                      350    32    743.2 500.2                                                      350    62    365.5 199.3                                                      350    92    166.8 93.5                                                       350    107   103.2 58.2                                                       350    122   67.7  38                                                         350    137   45.6  25.6                                 8:1:1      .6          85      3   307.0 134.2                                                      350    34    338.6 253.0                                                      350    64    364.7 235.1                                                      350    82    232.7 153.8                                                      350    94    166.6 107.7                                                      350    124   68.5  45.3                                                       350    149   38.1  28.5                                 8:1:1      1.0         80     3    434.0 180.1                                                      350    33    187.4 108.9                                                      350    52    127.2 71.5                                                       350    63    100.7 57.6                                                       350    78    74.4  41.7                                                       350    96    54.2  30.9                                                       350    108   43.7  24.9                                 ______________________________________                                    

These results clearly illustrate the effectiveness of the retardedcrosslinking compositions and the controlled delay that can be achievedby use of the composition of the present invention.

While particular embodiments of the invention have been described, it isto be understood that such descriptions are presented for purposes ofillustration only and that the invention is not limited thereto and thatreasonable variations and modifications, which will be apparent to thoseskilled in the art, can be made without departing from the spirit orscope of the invention.

What is claimed is:
 1. An aqueous gel comprising:an aqueous fluidcomprising a water-alcohol solution having from about 0 to 80 percentalcohol by volume; a gelling agent comprising a solvatablepolysaccharide having a molecular weight in excess of about 100,000selected from the group consisting of glucomannans, galactomannans, andderivatives thereof, said gelling agent being present in an amount offrom about 0.2 to about 1.25 percent by weight of said aqueous fluid,and a retarded crosslinking composition soluble in said aqueous fluidcomprising a zirconium IV salt or chelate and a polyhydroxyl-containingcompound having from about 3 to about 7 carbon atoms, the volumetricratio of said zirconium IV salt or chelate to saidpolyhydroxyl-containing compound being in the range of 0.01:1 to about100:1, said zirconium IV salt or chelate being present in an amount ofat least about 0.0005 percent by weight of said aqueous gel.
 2. Theaqueous gel of claim 1 wherein said gelling agent comprises solvatablepolysaccharides selected from the group consisting of guar gum, locustbean gum, karaya gum, sodium carboxymethylguar, hydroxyethylguar, sodiumcarboxymethylhydroxyethylguar, hydroxypropylguar, and sodiumcarboxymethylhydroxypropylguar.
 3. The aqueous gel of claim 1 whereinsaid crosslinking linking compound comprises at least one memberselected from the group consisting of zirconium (IV) acetylacetonate,zirconium lactate, zirconium carbonate and zirconium diisopropylaminelactate.
 4. The aqueous gel of claim 1 wherein saidpolyhydroxyl-containing compound comprises at least one member selectedfrom the group consisting of glycerol, erythritol, threitol, ribitol,arabinitol, xylitol, allitol, altritol, sorbitol, mannitol, dulcitol,iditol, and perseitol.
 5. The aqueous gel of claim 1 wherein saidcrosslinking compound, polyhydroxyl-containing compound and aqueousfluid of said retarded crosslinking composition are present in avolumetric ratio, respectively, in the range of from about 1:0.1:0.1 toabout 1:10:10.
 6. A process for fracturing a subterranean formationwhich comprises:introducing into said formation an aqueous gel at a flowrate and pressure sufficient to produce a fracture in said formation,said aqueous gel being comprised of an aqueous fluid, a gelling agentcomprising a solvatable polysaccharide having a molecular weight inexcess of about 100,000 present in an amount of from about 0.2 to about1.25 percent by weight of said aqueous fluid, a retarded crosslinkingcomposition soluble in said aqueous fluid of said aqueous gel comprisinga zirconium (IV) chelate or salt, a polyhydroxyl-containing compoundhaving from about 3 to about 7 carbon atoms and an alkanol or an aqueousfluid which are present in a volumetric ratio, respectively, in therange of from about 1:0.1:0.1 to about 1:10:10, said zirconium IV saltor chelate being present in an amount of at least about 0.0005 percentby weight of said aqueous gel, whereby the crosslinking rate of saidgelling agent by said crosslinking compound is retarded.
 7. The processof claim 6 wherein said gelling agent comprises a solvatablepolysaccharide selected from the group consisting of galactomannans,glucomannans, and derivatives thereof.
 8. The process of claim 6 whereinsaid gelling agent comprises solvatable polysaccharides selected fromthe group consisting of guar gum, locust bean gum, karaya gum, sodiumcarboxymethylguar, hydroxyethylguar, sodiumcarboxymethylhydroxyethylguar, hydroxypropylguar, and sodiumcarboxymethylhydroxypropylguar.
 9. The process of claim 6 wherein saidcrosslinking compound comprises at least one member selected from thegroup consisting of zirconium (IV) acetylacetonate, zirconium lactate,zirconium carbonate and zirconium diisopropylamine lactate.
 10. Theprocess of claim 6 wherein said polyhydroxyl-containing compoundcomprises at least one member selected from the group consisting ofglycerol, sorbitol, perseitol and mixtures thereof.
 11. The process ofclaim 6 wherein said aqueous fluid comprises a water-alcohol solutionhaving in a range of from about 0 to about 80 percent alcohol by volumeof solution.
 12. The process of claim 6 wherein said alkanol is selectedfrom alkanols having from 1 to 6 carbon atoms.
 13. The process of claim6 wherein said retarded crosslinking composition is present in the rangeof from about 0.01 gallon to about 0.5 gallon per 10 pounds of gellingagent.
 14. The process of claim 9 wherein said polyhydroxyl-containingcompound comprises at least one member selected from the groupconsisting of glycerol, erythritol, threitol, arabinitol, xylitol,allitol, altritol, sorbitol, mannitol, dulcitol, iditol and perseitol.15. The process of claim 9 wherein said retarded crosslinkingcomposition is aged for a period of a few minutes to several weeks. 16.A process for treating a subterranean formation penetrated by a wellbore which comprises:preparing a base gel by mixing an aqueous fluidwith a gelling agent selected from solvatable polysaccharides having amolecular weight of at least about 100,000, said gelling agent beingpresent in an amount of from about 0.2 to 1.25 percent by weight of saidaqueous fluid; admixing a retarded crosslinking composition soluble insaid aqueous fluid of said base gel with said base gel, said retardedcrosslinking composition comprising:(i) a crosslinking compoundcomprising a zirconium chelate or salt having zirconium in the ⁺ 4oxidation state, said zirconium chelate or salt being present in anamount of at least about 0.0005 percent by weight of said aqueous basegel, (ii) a polyhydroxyl-containing compound having from about 3 toabout 7 carbon atoms and (iii) an aqueous fluid present in said retardedcrosslinking composition in a ratio, respectively, in the range of fromabout 1:0.1:0.1 to about 1:10:10, said retarded crosslinking compositionbeing capable of delaying the reaction rate of said crosslinkingcompound with said gelling agent of said base gel; introducing said basegel containing said retarded crosslinking composition into said wellbore; permitting said base gel and said crosslinking compound to reactafter a controllable period of time to form a crosslinked aqueous gel,at least a portion of the delay in the rate of said reaction resultingfrom the presence of said polyhydroxyl-containing compound and aqueousfluid in said retarded crosslinking agent;and introducing saidcrosslinked aqueous gel into said formation from said well bore at aflow rate and pressure sufficient to fracture said formation.
 17. Theprocess of claim 16 wherein said polyhydroxyl-containing compoundcomprises at least one member selected from the group consisting ofglycerol, erythritol, threitol, arabinitol, xylitol, allitol, altritol,sorbitol, mannitol, dulcitol, iditol and perseitol.
 18. The process ofclaim 16 wherein said retarded crosslinking composition is aged for aperiod of a few minutes to several weeks.
 19. The process of claim 16wherein said retarded crosslinking composition is present in the rangeof from about 0.01 gallon to about 0.5 gallon per 10 pounds of gellingagent.
 20. The process of claim 16 wherein said crosslinking compoundcomprises at least one member selected from the group consisting ofzirconium (IV) acetylacetonate, zirconium lactate, zirconium carbonateand zirconium diisopropylamine lactate.